Mastery System Only Encourages Non-Team Play
It doesn't encourage more assists as it is weight MUCH less than kills, and it basically rewards you for big numbers, not good play. It doesn't reward team fight wins, objective control, tower kills, or even winning the game. You can have a lot of kills, lose, and still get an S+ (I know because I've done it).
It rewards you for playing for your score first and really only. Why do I get a B+ for having high cs with 1/0/5 but an S+ with that same score but like 8 kills?
Getting downvoted by people that like abusing their selfish play to get a high score over trying to win. Just stating facts, sorry if you don't like the truth.